1. Field of the Invention
This invention relates to a method, and the product produced thereby, for use in fabricating a multi-layered printed circuit board. It pertains, more particularly, to a printed circuit board having laminates which are formed from or secured by epoxy-resin layers having glass transition temperatures significantly higher than that of the epoxy-resin layers utilized to form or secure other laminates therein.
2. Description of the Prior Art
Multi-layered printed circuit boards are well known in the prior art. The processing technology therefor has advanced to the point where such boards are capable of supporting very high density circuits and modules. Even this capability, however, has not proven sufficient as advances in modern electronics have fueled a need for yet increased densities.
Increased density capability has been provided, in part, by increasing the number and size of signal, ground and voltage planes found in a multi-layered printed circuit board, as well as conductor density in the signal planes. In many applications, in order to minimize the number of interboard connections and the obvious potential problems associated therewith, it has become desirable to utilize as large and as dense a multi-layered circuit board as possible.
These design constraints and improved packaging techniques have placed greater, even acute, emphasis on dimensional stability. Clearly, as the size and circuit densities of a board are increased, it becomes increasingly difficult to maintain registration between the various interplanes since they have differing expansion characteristics which becomes important during the processing steps by which a particular board is fabricated.
In the process of forming a printed circuit board, it has been a standard industry practice to begin with a layer of an insulative base material, to circuitize one or both surfaces of the base material and to combine a number of these circuitized layers. Typical of the prior art processes are the methods shown and described in U.S. Pat. Nos. 3,523,037; 3,554,877; 3,791,858 and 3,867,759.
In U.S. Pat. No. 3,523,037, a typical epoxy composition is described as is its use in impregnating a glass fabric to form, after curing, the base material referred to above. In U.S. Pat. No. 3,554,877, individual, double-sided printed circuit boards are laminated together to form a multi-layered printed circuit board. Similarly, in U.S. Pat No. 3,867,759, a plurality of strip transmission line printed circuit board segments are used to form a plurality of printed circuit boards, each one of which becomes a layer in and part of a larger, integrated package. In U.S. Pat. No. 3,791,858, a laminated printed circuit board is produced using additive techniques for forming conductors within each layer with an initial layer being built up on an insulative base material, such as the glass cloth-epoxy resin combination referred to above.
In addition to the foregoing, U.S. Pat. Nos. 3,969,177 and 4,030,190 also disclose methods for forming improved multilayer printed circuit boards. U.S. Pat. No. 3,969,177 discloses a laminating method wherein a stack of cured and uncured, epoxy impregnated glass cloth layers are further cured at the same time without planishing plates. In U.S. Pat. No. 4,030,190, a method of forming dimensionally stable board assemblies is taught wherein each dielectric layer thereof is cured after being added to a base core.
Typically, such printed circuit boards are fabricated by forming a first layer from such an epoxy-glass material combination and curing it above its glass transition temperature whereat crosslinking takes place. Additional layers are added, after the base layer is circuitized. However, in forming such boards, a particular layer, once captured, may be subjected to several cycles above its glass transition temperature. Whenever this occurs, the linkage chains are thermally agitated and linkage is reduced. As a result, the affected layer or layers become rubbery, rather than glossy. The resultant deformation translates into a random dimensional change which seriously affects interlayer or interplane registration. The randomness of this dimensional instability makes predetermined compensation therefor rather difficult.